1. Technical Field
The present invention relates to a glossing device and an image forming apparatus incorporating the same, and more particularly, to a glossing device that processes a toner image with heat and pressure on a recording medium, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, which incorporates such a glossing capability.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process may be followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
Various techniques have been proposed to provide printing with high-gloss, photo-like imaging quality, several of which are directed to development of a more sophisticated fixing process.
Structurally, a fixing device with a glossing capability may be constructed of an endless rotary belt on which a recording medium is conveyed while subjected to heat and pressure. The endless belt is looped for rotation around multiple parallel rollers, including a heated roller and a stripper roller, with a pressure roller disposed opposite the heated roller via the belt to form a fixing nip therebetween.
During operation, a recording medium bearing a toner image, either unfixed or pre-fixed, thereon is conveyed through the fixing nip, which renders the incoming toner image into a semi-fluid, soft pliable adhesive state under heat and pressure. After passage through the fixing nip, the recording medium is conveyed with the toner image adhering to the belt, which imparts gloss to the toner image as the molten toner gradually cools and solidifies while conforming to the smooth surface of the belt. The recording medium closely contacts the belt as the belt moves from the heated roller toward the stripper roller, and separates from the belt as the belt passes around the separator roller.
To date, belt-based fixing devices are designed with a belt cooler for cooling an endless rotary belt during conveyance of a recording medium downstream from a fixing nip, so as to provide efficient, uniform cooling of the recording medium to a desired temperature after fixing and glossing a toner image thereon.
For example, one known technique proposes a dual-mode glossing device for processing a toner image in a high-gloss mode or a low-gloss mode using an endless belt, which employs a pair of cooling devices, one disposed inside and the other outside the loop of the endless belt, to cool the belt and the recording medium in contact with the belt. The paired cooling devices may be electric fans that remove heat by directing an air flow to the belt, or those that employ a thermally conductive member, such as a heat pipe or heat sink, containing water or liquid coolant flowing therethrough to absorb heat from the belt through contact with the thermally conductive member.
Another known technique proposes a fixing system including a thermal pre-fixing unit and a gloss adjustment unit for adjusting glossiness of the toner image using an endless belt, which employs a cooling device disposed inside the loop of the endless belt to cool a toner image on the recording medium being conveyed. The cooling device includes a heat dissipator or heat sink disposed in contact with the belt to absorb heat from the belt. The heat dissipator may be used in combination with a cooling fan disposed outside the loop of the belt, which assists in cooling the belt by directing an air flow to the belt.
Still another known technique proposes a copying system including a gloss detector for measuring glossiness of an original document, and a belt-based fixing device for adjusting gloss of a copied image according to the measured gloss of the original, which employs a cooling device disposed outside the loop of the endless belt to cool the belt to a variable, adjustable temperature. The cooling device includes a cooling fan that operates at an adjustable flow rate to control the temperature of the belt according to readings of the gloss detection unit, so as to provide the resulting print with a high-gloss or low-gloss appearance similar to that of the original document.
Yet still another known technique proposes a belt-based fixing device that can control an amount of compression experienced by the belt upon cooling, which employs a cooling device disposed inside the loop of the endless belt to cool the belt to a desired temperature. The cooling device includes multiple cooling members of different cooling capacities disposed in thermal contact with the belt, which are arranged with respect to each other in a longitudinal, conveyance direction of the belt such that those located upstream have lower heat capacities than those located downstream for preventing the belt from a rapid temperature change and a concomitant thermal contraction during cooling.
Although generally successful for their intended purposes, the approaches depicted above have several drawbacks.
For example, the belt cooler employed in those belt-based fixing devices is vulnerable to reduced efficiency where a large number of print jobs are sequentially processed. Sequential processing of print jobs often results in substantial amounts of heat released to the surrounding over time. In case of air-cooled, non-contact cooling that employs a cooling fan, heat released to the surrounding air translates into a heated air flow generated by the cooling fan, and a concomitant rise in temperature of the belt. In case of a contact cooling system or heat sink that directly contacts an endless rotary belt to absorb heat from the belt, heat released during sequential processing of print jobs gradually heats the heat sink, which then no longer works to remove heat from the belt as efficiently as intended.
Failure to properly cool the belt to a desired temperature results in failure to provide printing with high-gloss, photo-like imaging quality. The problem is particularly pronounced in high-speed printing applications where the endless belt rotates at a relatively high processing speed, which translates into a reduced duration of time during which the belt is subjected to cooling within a single operational cycle.